1. Field of the Invention
The present invention relates to the field of leak detection systems, and more particularly to a method and apparatus for detecting sodium leaks from a liquid metal cooled nuclear reactor.
2. Art Background
In a nuclear power reactor, power is generated through heat emanating from the nuclear core. This core must be kept within a certain temperature range or it may melt through the surrounding reactor vessel and damage the outside environment. One method of cooling a reactor core involves the use of water. The water circulates around the core and is heated to steam. This steam is passed through a turbine creating electric power. The turbine also cools the steam and condenses it back into water. This cool water is then recirculated around the core in a closed cycle.
Another method of cooling the reactor core involves the use of liquid metal. Typically, liquid sodium is chosen because of its superior material properties. The use of liquid sodium has many advantages over water cooled reactors. The use of sodium decreases the probability of neutrons-which are generated by the core-slowing down, and thus increases the average neutron kinetic energy and reduces the neutron absorption. All of these factors increase the efficiency with which the reactor produces power. The pool of liquid sodium also acts as a large heat sink, providing thermal stability to damp out any temperature oscillations in the reactor system. In addition, the excellant heat transfer properties of sodium results in an improved thermal efficiency for the reactor's power conversion system.
The core and the pool of liquid sodium are contained within a large reactor vessel. The reactor vessel is in turn concentrically surrounded by a separate containment vessel. In between the reactor and containment vessels is an airtight annular gap. This gap is filled with an inert gas such as argon.
It is exceedingly important that the integrity of the reactor vessel be maintained. If the reactor vessel is breached, the radioactive core material may be released into the outside environment. This core material is exceedingly hazardous. It is therefore essential that any fractures in the reactor vessel be quickly discovered. In a liquid sodium type reactor, fractures in the reactor vessel may be discovered by monitoring the amount of sodium which is present in the annular gap between the reactor and containment vessels. Since the gap is normally filled exclusively with an inert gas, any sodium that may be present must come from within the reactor vessel. This can only occur if the reactor vessel's structural integrity has been compromised. Monitoring the concentration of sodium level within the gap will lead to the detection of any cracks in the reactor vessel.
There are many events which may lead to rupture of the reactor vessel. Some of these events, such as an earthquake, present an obvious need to inspect the reactor vessel's integrity. Other causes of damage are more subtle. Even at low reactor power, with low pressure on the liquid sodium and with negligible pressure induced stresses, the high radiation environment and high temperature environment along with local temperature gradients may lead to the formation of cracks. These events can occur at any time, without advance warning or any overt indications beforehand. Periodic monitoring for cracks is unacceptable since the cracks may form between the inspection periods. It is therefore desirable to have an efficient method of continuously monitoring the amount of sodium present in the annular gap.
In prior art systems, the concentration of sodium in the gaseous media was measured intrusively. For example, sodium atoms may be ionized in the gas and the concentration of sodium ions measured by physically removing samples from the annular gap. The presence of sodium may also be detected by monitoring the physical properties such as thermal conductivity, electrical conductivity and density, of the gas at various locations within the gap. However, all of these techniques for the detection of sodium leaks are undesirable as they require breaches in the containment vessel, or placement of active sensors within the vicinity of the reactors. All of these methods introduce additional safety hazards to an already dangerous environment.